6.5 seized while troubleshooting fuel system

[Sparky34]

My 1996 6.5 TD Suburban 2500 ran fine when I parked it...next morning it turned over fine but wouldn't start. Got a fairly detailed fuel system troubleshooting guide online and went to work. Confirmed that fuel made it to the injector pump, but not to the injectors. Got most of the way through the troubleshooting process (just the PMD and IP left) when the motor seized hard while turning over with the starter. No warning, just "clunk". Any thoughts? Is there something in the IP coupling that could fail and cause this? Thanks!

[trbankii]

How are your batteries? It wouldn’t be the first time I heard of someone running the batteries down while troubleshooting.

[trbankii]

And when you say seized - have you tried turning it by hand with a wrench on the front bolt?

[svcattle]

I'm going to say batteries as well. also possible you cooked the starter

[a5150nut]

If by seized you mean it suddenly quit rotating I would first try turning by hand with a wrench. If it is indeed seized, pull all the glow plugs and try again. If you get fuel or water out the glow plug openings you have some serious internal damage. If you continue to try turning it with the starter you will increase the amount of damage.

[Sparky34]

Thanks for the replies. Definitely not the batteries...both are new and fully charged (the starter is also new), and the motor was never cranked for more than 5 seconds at a time...just long enough to either take voltage readings at specified check points or to show fuel from the bleeder. I engaged the starter to check signals at the PCM and it spun the motor vigorously, then "clank". After that, turning the key resulted in a very healthy engagement of the starter solenoid and, a couple of times, a heavy metallic clank from the motor, but no motion. It sounded definitely like something is jammed up in there. The PCM is fairly new and remotely mounted on a large heat sink, but there was no fuel being delivered to the injectors...that's what got me thinking about the IP. I haven't tried turning the crank by hand yet (i'll try that tomorrow), but did relax the serpentine belt tensioner and checked that all the ancillaries are turning freely.